Welcome to a special longer edition of the Scientific American podcast 60-Second Earth:

"This is Mike Weber. We received a cable through international programs which came from the Ambassador in Vienna, and we just want to alert you to this, not that you need to do anything with the information, but to make certain that you have awareness of it."

Mike Weber is with the U.S. Nuclear Regulatory Commission, the NRC. And that's him on March 11, 2011, a year ago today, informing his colleagues at the agency that a nuclear accident is brewing in Japan following the 9.0-magnitude earthquake and tsunami. This recording and the others that followed come from those released by the NRC in the wake of multiple Freedom of Information Act requests. Weber went on:

"It's a brief cable that summarizes the current perspective of the International Atomic Energy Agency with respect to the situation of nuclear power plants in Japan.

And there is some somewhat alarming language that talks about, and I'll just quote 'the IAEA tells us the earthquake triggered a power failure at the Fukushima Daiichi unit 2 nuclear power plant, and then when a backup generator also failed, the cooling system was unable to supply water to cool the reactor.

Specialists at the IAEA understand the fuel core is still covered by water, but they question if it will remain so.'"

In fact, there would be at least three meltdowns at Fukushima Daiichi before it was all over. The crisis ultimately required the potentially permanent evacuation of hundreds of thousands of people in a 20-kilometer radius around the nuclear complex. NRC chairman Gregory Jaczko wondered how bad it might get on March 12. Responding to him is Dan Dorman, an NRC official.

JACZKO: "Where do you think we are, in terms of getting now, an assessment that includes an unvented or an open containment?"

DORMAN: "We have an assessment that is 100 percent core melt in an open containment and that's, as you would imagine, a very bad scenario with reaching the protective action guidelines out to 50 miles down wind."

Those guidelines in the U.S. are to prevent a significant dose of radiation, roughly 10 millisieverts in a short amount of time. The NRC allows nuclear power plant employees to endure a total of 50 millisieverts of radiation per year. Immediate radiation sickness hits at around 2000 millisieverts.

By March 14, hydrogen gas explosions captured live on television had blown out the walls and roofs of the Unit 1 and 3 reactor buildings. And then there was a loud sound from Unit 2, prompting this exchange between first NRC officials Jack Grob in Maryland and then Tony Ulses in Japan.

GROB: "Hey guys, when you said a loud sound, what did you interpret that as?"

ULSES: "It was my, I think both of us believe that one of the sources, that it wouldn't, it wasn't like the other two loud sounds with Unit 1 and 3 when the reactor building blew.

You know, my guess is, and it's just, it's just pure conjecture, would be it was probably when the core went ex-up."

Ex-up, or ex-vessel, means, in the jargon of the nuclear trade, melted down nuclear fuel that has burned its way out of the reactor.

UNIDENTIFIED: "And landing in the water under the vessel, it would have caused a little steam explosion."

GROB: "That's what I heard you say. But what you think is that that was a steam explosion from the fuel going ex-vessel, and we heard that containment at that point in time went from three atmospheres to one atmosphere."

ULSES: "That makes sense."

Normally, a nuclear reactor operates at pressures far greater than the atmospheric pressure outside. So when the pressure in a nuclear reactor becomes the same as the surrounding environs, it suggests that reactor has a hole in it.

At this point, it was unclear exactly what the condition is of the melted down nuclear fuel in Units 1, 2 and 3. Radiation levels remain too high for a proper assessment. And the crisis would get worse as Grob reported on March 14.

GROB: "Unit 4 previously was reported as stable. That is no longer the case. The unit 4 reactor is de-fueled. The spent fuel pool is in --excuse me-- the spent fuel is in the spent fuel pool. The spent fuel pool is dry and there appears to be a zirconium fire in the spent fuel pool of Unit 4."

And worse.

GROB: "Unit 2 is not in very good condition at all. It appears that core cooling has not existed for quite some time. It appears that the pumps that were injecting into the core have been deadheaded for some time. Several hours ago, there was a loud sound that appeared to come from inside the dry well, and at that time, the pressure inside primary containment went from 3 atmospheres to one atmosphere, essentially atmospheric pressure. There is clear indication that primary containment is not intact, and there may also be indication that fuel, that there has been substantial core melt, and possibly even the loud sound in the breach of containment was caused by an ex-vessel fuel situation."

The consequence of such a melt down could quickly spiral out of control, thanks to the older safety system design in use at Fukushima Daiichi, known as a Mark I containment. This structure, which rings the reactor itself at the bottom, has been known to be flawed for decades—as the NRC's Charles Casto, who flew to Japan to assist, outlined on March 16, discussing the NRC's safety guidelines for this type of reactor or NUREG.

CASTO: "You know, if we end up with a molten core and then you talk about the time for the concrete to disassociate, you know, that NUREG says it's a couple of inches an hour, you know. And, of course, that Mark I containment is the worst one of all the containments we have, and it's literally, you know, this NUREG tells you that in a station blackout you're going to lose containment. There's no doubt about it."

No electricity to run pumps to add cooling water to the melting down reactors. Rubble from the earthquake, tsunami and multiple explosions. Hazardous radiation levels throughout the facility preventing access to vital areas, such as the control rooms that allow workers to operate the nuclear power plant's systems. NRC official John Monninger:

MONNINGER: "I mean, they're, they run in the control rooms back and forth occasionally, but they're not really manning the control rooms."

Plus, the Japanese had to resort to pumping seawater into the reactors in a bid to cool them. The boiling hot nuclear fuel evaporated the brine, leaving salt to pile up inside. And that made the Japanese nervous. The NRC's Casto:

CASTO: "Could you please explain your concern with the seawater on the reactor vessel? And they said, we are very concerned with seawater deposition and impacts on the bottom heads, salt's impacts on the bottom heads, and we're very concerned about heat transfer rate on the fuel."

With good reason, according to NRC official Brian Sheron, talking with Casto in Japan:

SHERON: "We can get you some articles that we have here on salt. We've got one here from Naval Reactors. The punch line in there is that they should probably switch over to freshwater sources, if they have it, as soon as possible."

CASTO: "Yeah, and we talked about that, and I said you need to run your desalinization plant or reverse osmosis. And the Navy here is telling me, I don't know. The Navy guys can explain more about that stuff than I know about it, but we talked about that, and they're prepared to do that.

The challenges, as you well know, are how to do it. I mean, they can make freshwater, but the challenge is: how do you get freshwater into the reactors, you know, with a 100-R field or a 450-R field?"

Fukushima Daiichi could have been much, much worse, as Commissioner Jaczko observed on March 16.

JACZKO: "This is going to progress to the point at which we probably have, we, I think, have to assume at this point that we're going to have three reactors out of control and possibly up to six spent fuel pools."

In fact, a plume of radioactive contamination would spread for kilometers around the stricken nuclear power plant, shifting with the wind and weather. NRC official Patricia Holahan:

TRISH HOLAHAN: "Okay, the ANS flyover found the greatest concentration of contaminated materials to the northwest of the site, and there's a narrow band to the northwest beyond 13 miles from the site where the integrated four-day doses approach or exceed 1 rem. So we made the right decision in our prior recommendations in evacuating everybody within 50 miles."

Thanks to such evacuations and the heroic efforts of Japanese emergency workers, however, no one has died as a result of the nuclear crisis at Fukushima Daiichi. Tragically, the same cannot be said for the earthquake and tsunami that started it, as the NRC's Casto noted on March 17.

CASTO: "I mean these people have got--it's hell over here for that government. I mean, it's just absolutely hell. And I know we get frustrated with them, but, man, when you think about what they're faced with, it's absolutely unfathomable.

When you've got a thousand dead bodies washing up on shore, you know, it's, you know, these people, I don't know, I mean, we, you know, we think we'd be prepared for it, but it's, this is, it is tough for them. And we're over here barking at them about--and I want to do it, don't get me wrong. I'm not trying to --- but you know, we do have to be a little patient. They're working hard and they have a lot of challenges."

A year later, all the nuclear fuel is now cooled enough that it can no longer boil water, or melt down. Japan's greenhouse gas emissions have swelled as it burns fossil fuels to replace the lost nuclear power. And the grieving continues for the tens of thousands who lost their lives in the tsunami as well as the tens of thousands who may never return home to the cities, towns and villages too close to the stricken nuclear power plant known as Fukushima Daiichi.